IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 50, NO. 2, FEBRUARY 2002 235 Chip-Interleaved Block-Spread Code Division Multiple Access Shengli Zhou, Student Member, IEEE, Georgios B. Giannakis, Fellow, IEEE, and Christophe Le Martret Abstract—A novel multiuser-interference (MUI)-free code di- suppress MUI statistically (except for the ZF option) even vision multiple access (CDMA) transceiver for frequency-selective with exact channel state information (CSI). In addition to multipath channels is developed in this paper. Relying on chip-in- increased complexity that comes with multichannel estimation terleaving and zero padded transmissions, orthogonality among different users’ spreading codes is maintained at the receiver even and multiuser detection, there even exist frequency-selective after frequency-selective propagation. As a result, deterministic channels preventing symbol detection no matter what receiver multiuser separation with low-complexity code-matched filtering is used (see [10] for illuminating counter-examples). becomes possible without loss of maximum likelihood optimality. To remove MUI deterministically regardless of the un- In addition to MUI-free reception, the proposed system guar- derlying multipath channels, several alternatives have been antees channel-irrespective symbol detection and achieves high bandwidth efficiency by increasing the symbol block size. Filling proposed recently. Those include the orthogonal frequency the zero-gaps with known symbols allows for perfectly constant division multiple access (OFDMA) [22] (and generalizations modulus transmissions. Important variants of the proposed [25]), where complex exponentials are utilized as informa- transceivers are derived to include cyclic prefixed transmissions tion-bearing subcarriers that retain their orthogonality when and various redundant or nonredundant precoding alternatives. passing through multipath channels. However, when the (Semi-) blind channel estimation algorithms are also discussed. Simulation results demonstrate improved performance of the channels have nulls (or deep fades) on some subcarriers, the proposed system relative to competing alternatives. information symbols on those subcarriers will be lost. There- fore, OFDMA-like transceivers require extra diversity (such Index Terms—Block spreading, channel estimation, chip inter- leaving, code division multiple access (CDMA), multipath, mul- as frequency hopping or channel coding) to ameliorate fading tiuser interference. effects. To guarantee channel-irrespective MUI-free reception and symbol detection, a mutually-orthogonal usercode-receiver (AMOUR) system was proposed in [11]. Subsequently, a I. INTRODUCTION generalized multi-carrier (GMC) CDMA was developed in ELYING on orthogonal spreading codes, code division [10], [27] that also unifies many existing schemes. However, R multiple access (CDMA) systems enable simultaneous similar to all multicarrier systems, AMOUR transmissions are transmissions from multiple users over the same bandwidth not constant-modulus (C-M) in general, even though a special and time duration. However, when the chip rate increases, the code design exhibits C-M, and will turn out to fall under the underlying multipath channel becomes frequency selective; it system designs developed herein. AMOUR codes are generally introduces inter chip interference (ICI), and thus destroys code complex valued and bandwidth efficiency drops by 50% if real orthogonality at the receiver. The latter gives rise to multiuser codes are to be designed from complex ones [27]. To maintain interference (MUI). To suppress MUI, various multiuser C-M at the transmitter and facilitate low-complexity receivers detectors are available [26], e.g., the linear decorrelating or for MUI-free reception, the so-called shift orthogonal codes zero forcing (ZF), the minimum mean square error (MMSE), (which are not only orthogonal to each other but also to their as well as the nonlinear decision feedback (DF) and maximum shifted versions) were proposed in [15] and [16]. However, likelihood (ML) receivers. However, these schemes require to maintain shift orthogonality, a 50% bandwidth efficiency knowledge of the multipath channels for all users and/or penalty is paid for both the real and the complex codes in [15] and [16]. In this paper, we develop novel MUI-free CDMA transceivers Paper approved by W. A. Krzymien, the Editor for Multicarrier and Spread based on a specific block-spreading operation, which can be Spectrum Systems of the IEEE Communications Society. Manuscript received August 31, 2000; revised June 9, 2001. This work was supported by the Na- viewed as (and is implemented by) symbol-spreading followed tional Science Foundation under Wireless Initiative Grant 99-79443 and NSF by chip interleaving. The resulting so-called chip interleaved Grant 0105612. This paper was presented in part at the 38th Allerton Confer- block spread (CIBS) transceivers are applicable to both uplink ence, University of Illinois at U-C, Monticello, IL, October 4–6, 2000, and at the 35th Conference on Information Sciences and Systems, the Johns Hopkins and downlink scenarios. Thanks to chip-interleaving and zero University, Baltimore, March 21–23, 2001. padding at the transmitter, mutual orthogonality between S. Zhou and G. B. Giannakis are with the Department of Electrical and different users’ codes is preserved even after multipath prop- Computer Engineering, University of Minnesota, Minneapolis, MN 55455 USA (e-mail: [email protected]; [email protected]). agation, which enables deterministic multiuser separation C. Le Martret was with the Department of Electrical and Computer through low-complexity code-matched filtering without loss Engineering, University of Minnesota, Minneapolis, MN 55455 USA, on of maximum likelihood optimality. Consequently, multiuser leave from the Centre d’Électronique de L’ARmement (CELAR), 35170 Bruz, France (e-mail: [email protected]). detection is successfully converted to a set of equivalent Publisher Item Identifier S 0090-6778(02)01360-0. single-user equalization problems (Section III). In addition to 0090–6778/02$17.00 © 2002 IEEE 236 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 50, NO. 2, FEBRUARY 2002 Fig. 1. Continuous and discrete-time equivalent system model. MUI-free reception, channel-irrespective symbol detection is , where is the also guaranteed. Since the only requirement is mutual orthog- chip period and is the chip pulse. The th user’s onality among users’ codes, the code design is very flexible transmitted signal propagates through a (possibly and enables fast algorithms at the receiver. By increasing the unknown) channel and is filtered by the receive filter symbol block size, the proposed system achieves high band- that is matched to . Let be the convolution of width efficiency. Perfectly constant modulus transmissions transmit- with receive-filters. With denoting convolution, let become available by filling the zero gaps with known symbols. Variants of the proposed transceivers are also developed in be the chip-sampled discrete time equivalent FIR channel Section IV to include cyclic prefixed (CP) transmissions as well corresponding to the th user. The FIR channel of order as various (redundant or nonredundant) precoded and loaded includes the th user’s asynchronism in the form of delay transmissions. Transforming the multiple-input multiple-output factors as well as transmit–receive filters and frequency-selec- (MIMO) channel to an equivalent set of parallel single-input tive multipath effects. With denoting single-output (SISO) channels implies that the receiver can sampled noise, the aggregate received sequence from all employ various training-based or (semi-) blind channel estima- users at the chip rate can then be written as tors developed for single user systems (Section V). Simulations are then performed in Section VI and concluding remarks are drawn in Section VII. (1) Notation: Bold upper (lower) letters denote matrices (column vectors); and denote transpose and Hermi- Similar to [22], [25], [11], and [15], we here focus on a prac- tian transpose, respectively; and stand for Kronecker’s tical quasi-synchronous (QS) system in the uplink, where the delta and Kronecker product, respectively. for expecta- mobile users attempt to synchronize with the base-station’s pilot tion, for integer ceiling; denotes the identity matrix of waveform and have a coarse common timing reference. A good size ; denotes an all-zero (all-one) matrix example is the European 3rd Generation mobile communica- with size ; and, denotes a FFT matrix with tion system based on hybrid TDMA/CDMA (briefly denoted as th entry . T-CDMA), where multiple users per cell are allowed to transmit over the same time slot [1]. Asynchronism among users is thus II. SYSTEM MODELING limited to only a few chip intervals; the maximum asynchro- The block diagram in Fig. 1 describes a CDMA system model nism arises between the nearest and the farthest mobile in either uplink or downlink operation, where only one user users within the cell, and can be predetermined from the radius (the th user out of a maximum users) is shown. Unlike of the cell and the adopted chip interval . With de- traditional spreading which is performed over a single symbol, noting maximum multipath spread, which is found using field we here use block spreading that operates on a block of sym- measurements from the operational environment, the maximum bols; block spreading has also been used in, e.g., [27], [11], channel order